A broadband wireless access system defines protocols of a media access control (MAC) layer and a physical (PHY) layer for point-to-multipoint connection between a base station and a mobile station. A protocol layer structure of the broadband wireless access system is as shown in FIG. 1. The uppermost of the MAC layer is a service specific convergence sublayer, and serves to convert packet data of various core networks of high level into common protocol data unit (PDU) data according to MAC standards and compress a header of a corresponding packet.
FIG. 2 illustrates procedures of performing network entry when a mobile station is initiated in an orthogonal frequency division multiple access (OFDMA) type broadband wireless access system according to the related art.
1. If a mobile station is initially turned on, the mobile station retrieves a downlink channel and receives downlink/uplink map message (DL/UL MAP) and downlink/uplink channel descriptor messages (DCD, UCD) to acquire synchronization with a base station.
2. If the mobile station receives uplink map message having allocation information of an initial ranging region, the mobile station selects one of initial ranging codes and transmits the selected ranging code to the base station by using the initial ranging region. The base station which has received the initial ranging codes notifies the mobile station of uplink transmission parameter adjustment values such as time offset, frequency offset and power offset through a ranging response message (RNG-RSP), and the mobile station which has received the uplink transmission parameter adjustment values adjusts uplink transmission parameters. The mobile station and the base station repeat such exchange procedures of the ranging codes and the ranging response message to allow the mobile station to perform uplink transmission parameter adjustment. If the uplink transmission parameter adjustment is successfully performed, the base station forwards a success code to a corresponding mobile station along with the ranging response message. At this time, the base station allocates non-contention based uplink resources to the corresponding mobile station through a specific uplink map information element (i.e., CDMA allocation UL-MAP IE) to allow the mobile station to transmit a ranging request message. The mobile station transmits a ranging request message having its MAC address by using the allocated uplink resources, and the base station which has received the ranging request message transmits a ranging response message (RNG-RSP) for allocation of a basic connection identifier (basic CID) and a primary CID to the corresponding mobile station.
3. The mobile station which has assigned with the basic CID and the primary CID through the ranging response message transmits to the base station SS basic capability request (SBC-REQ) message for negotiation of basic capability with the base station, and receives SS basic capability response (SBC-RSP) message from the base station. To transmit the SBC-REQ message, the mobile station should be allocated with an uplink band from the base station. To this end, the mobile station receives bandwidth request allocation region information from the base station through the uplink map, and transmits a randomly selected bandwidth request code to a corresponding uplink region. The base station which has received the bandwidth request code from the mobile station allocates the non-contention based uplink resources to the corresponding mobile station through the specific uplink map information element (CDMA allocation UL-MAP IE), and the mobile station which has received the non-contention based uplink resources transmits a bandwidth request header for requesting an uplink band required for transmission of the SBC-REQ message by using a corresponding uplink resource. The base station which has received the bandwidth request header allocates the uplink resource to the corresponding mobile station through the uplink map information element (UL-MAP IE). The mobile station transmits the SBC-REQ message by using the allocated uplink resource to forward its basic capability information to the base station. The base station which has received the SBC-REQ message forwards the SBC-RSP message to the corresponding mobile station to complete basic capability negotiation procedures between the mobile station and the base station. After transmitting the bandwidth request code to the base station, if the mobile station is assigned with the uplink resource that can transmit the SBC-REQ message, the mobile station can directly transmit the SBC-REQ message to the base station without transmission of the bandwidth request header.
4. The mobile station which has received the SBC-RSP message exchanges a privacy key management request (PKM-REQ) message and a privacy key management response (PKM-RSP) message for authentication and key exchange procedures with the base station. At this time, the mobile station may perform the bandwidth request code transmission of the procedure 3 above to request uplink resource allocation for transmission of the PKM-REQ message.
5. If the authentication and key exchange procedures between the mobile station and the base station is completed through exchange of the PKM-REQ message and the PKM-RSP message, the mobile station exchanges a registration request (REG-REQ) message and a registration response (REG-RSP) message with the base station to perform a registration procedure to the base station. At this time, in the same manner as the procedures 3 and 4, the mobile station may transmit the bandwidth request code to the base station to request the uplink band for transmission of the REG-REQ message.
6. The mobile station and the base station, which have completed the registration procedure through exchange of the REG-REQ message and the REG-RSP message, perform a procedure for setting Provisioned Service Flow of a network. ‘Provisioned Service Flow’ is performed in such a manner that the base station transmits a dynamic service addition request (DSA-REQ) message having a service quality related parameter and the mobile station forwards a dynamic service addition response (DSA-RSP) message to the base station in response to the DSA-REQ message. Simultaneously with completing setting of ‘Provisioned Service Flow,’ the mobile station and the base station complete an initial network registration procedure and are converted into to a normal operation status.
According to the related art, the number of several timers and the number of retransmission times are prescribed so that the mobile station can normally exchange a management message with the base station during initial network entry. For example, a timer T3 which waits for reception of the ranging response message from the base station is defined in the mobile station which has transmitted the ranging request message, and the mobile station retransmits the ranging request message by the number of retransmission times previously defined if the mobile station does not receive the ranging response message until a corresponding timer expires. If the mobile station does not receive the ranging response message from the base station until the number of retransmission times of the ranging request message exceeds the number of prescribed retransmission times, the mobile station determines abnormal status and tries to access another base station or restarts the initiation procedure for access to the corresponding base station.
According to the related art, the Parameters relating to the number of timers and the number of retransmission times required for the initiation procedure of the mobile station are prescribed as shown in Table 1.
TABLE 1MinimumDefaultMaximumSystemNameTime referencevaluevaluevalueBSInitial RangingTime between Initial2 sIntervalRanging regions assignedby the BSSSContentionNumber of retries on16Ranging Retriescontention RangingRequestsSS, BSInvited RangingNumber of retries on16Retriesinviting Ranging RequestsSSRequest RetriesNumber of retries on16bandwidth allocationrequestsSSRegistrationNumber of retries on 3Request Retriesregistration requestsBSTprocTime provided betweenSC: 200 μsarrival of the last bit of aOFDM:UL-MAP at an SS and1 mseffectiveness of that mapOFDMA:10OFDMAsymbolsBSSS RangingTime allowed for an SS10 msResponsefollowing receipt of aProcessing Timeranging response before itis expected to reply to aninvited ranging requestSS, BSDSx RequestNumber of Timeout3RetriesRetries onDSA/DSC/DSD RequestsSS, BSDSx ResponseNumber of Timeout3RetriesRetries onDSA/DSC/DSDResponsesSST1Wait for DCD timeout5 * DCDintervalmaximumvalueSST2Wait for broadcast5 *ranging timeoutrangingintervalSST3Ranging Response200 ms200 ms reception timeoutfollowing thetransmission of a RangingRequestSST4Wait for unicast ranging 30 s35 s opportunity. If thepending-until-completefield was used earlier bythis SS, then the value ofthat field shall be added tothis interval.BST5Wait for Uplink Channel2 sChange responseSST6Wait for registration3 sresponseSS, BST7Wait for DSA/DSC/DSD1 sResponse timeoutSS, BST8Wait for DSA/DSC300 ms Acknowledge timeoutBST9Registration Timeout, the300 ms300 mstime allowed between theBS sending a RNG-RSP(success) to an SS, andreceiving a SBC-REQfrom that same SSSS, BST10Wait for Transaction End3 stimeoutSST12Wait for UCD descriptor5 * UCDIntervalmaximumvalueSST14Wait for DSX-RVD200 ms TimeoutSST18Wait for SBC-RSP 50 ms<<T9timeout
Table 2 illustrates an example of initial ranging backoff window start parameter and initial ranging backoff end parameter which are included in an uplink channel descriptor message, wherein the parameters are used as minimum and maximum values of backoff window during initial ranging of the mobile station.
TABLE 2Type(1Namebyte)LengthValueInitial_ranging_backoff_start1981Initial backoff window size for initial rangingcontention, expressed as a power of 2. Values of nrange 0-15 (the highest order bits shall be unusedand set to 0) This TLV shall be used in NBR-ADVmessage only to represent corresponding values thatappear in UCD message fields.Initial_ranging_backoff_end1991Final backoff window size for initial rangingcontention, expressed as a power of 2. Values of nrange 0-15 (the highest order bits shall be unusedand set to 0) This TLV shall be used in NBR-ADVmessage only to represent corresponding values thatappear in UCD message fields.
FIG. 3 is a block diagram illustrating the operation when random backoff is used during initial ranging of the mobile station.
Hereinafter, an example of transmitting initial ranging codes from the mobile station when an initial ranging backoff value is set to 13 during initial ranging of the mobile station will be described with reference to FIG. 3. In this case, since the mobile station randomly selects a backoff value within an initial backoff window, the size of the initial back off window in Table 1 should be set to be greater than 13.
When the mobile station selects the initial ranging backoff value of 13 as shown in FIG. 3, the mobile station transmits a ranging code to a fourteenth uplink ranging slot. In this way, the mobile station randomly selects the ranging slot for transmitting the ranging code within the ranging backoff window so as to minimize collision that may occur as several mobile stations simultaneously transmit the ranging code.
Furthermore, according to the related art, when the base station is restarted during normal operation, mobile stations recognize restart of the base station and perform initial network entry to the base station. To this end, the base station forwards the number of restart times of the base station to the mobile station through a downlink channel descriptor (DCD), and the mobile station, which has received the number of restart times, determines whether to perform initial network entry by determining whether the base station is restarted.
Table 3 is an example of a base station (BS) restart counter parameter, which is included in the downlink channel descriptor (DCD).
TABLE 3TypePHYName(1 byte)LengthValue (variable length)scopeBS1541The value is incremented by oneAllRestartwhenever BS restarts (see 6.3.9.11).CountThe value rolls over from 0 to 255.
The base station increases a BS restart count parameter setting value by 1 as shown in Table 3 whenever restart is performed. After receiving the downlink channel descriptor message, the mobile stations compare a previous BS restart count value with the current value and determine whether the base station has been restarted. If the base station has been restarted, (i.e., if the BS restart count value is increased as compared with the previous BS restart count value) the mobile stations are again registered in the base station by performing initial network entry.
FIG. 4 illustrates problems that may occur between the initial ranging procedure and the basic capability negotiation procedure according to the related art.
As shown in FIG. 4, the mobile station is allocated with the basic CID and the primary CID from the base station through the initial ranging procedure. Then, the mobile station transmits the SBC-REQ message to the base station to initiate the basic capability negotiation. Also, the base station transmits the ranging response message to the mobile station to allocate the basic CID and the primary CID to the mobile station and then operates a timer T9 to wait for the SBC-REQ message from the mobile station. If the base station does not receive the SBC-REQ message until the timer expires, the base station releases the basic CID and the primary CID allocated to the mobile station, and the same CIDs can be allocated to another mobile station.
The mobile station should be allocated with uplink resources from the base station to transmit the SBC-REQ message to the base station. To this end, the uplink bandwidth request procedure is required. However, in this case, the uplink bandwidth request procedure may collide with another uplink bandwidth request procedure of another mobile station or the base station may not receive the uplink bandwidth request from the mobile station due to deterioration of channel quality. For this reason, the uplink bandwidth request may be retried. In other words, if the mobile station does not transmit the SBC-REQ message as the timer which is being operated within the base station expires in a state that the mobile station is not allocated with the uplink resources, the base station releases the CIDs allocated to the mobile station, and the mobile station may not recognize that the CIDs are released. If the base station allocates the corresponding CID to another mobile station, a problem may occur in that the same CID may be allocated to two different mobile stations.
FIG. 5 illustrates problems that may occur between the base station registration procedure and the ‘Provisioned Service Flow’ procedure when the mobile station is initiated in the related art.
The mobile station and the base station complete network entry procedure of the mobile station by completing the provisioned service flow procedure after the registration procedure is completed. However, as shown in FIG. 5, if the mobile station does not receive a service flow request message for a certain time period after receiving a registration response message from the base station, the mobile station is not converted into normal operation and gets into error status. Also, if the base station which has transmitted the service flow request message to the mobile station does not receive the service flow response message for a certain time period, the base station retransmits the service flow request message. If the base station does not receive the service flow response message even after transmitting the service flow request message by the number of given times, the base station becomes abnormal status. The related art fails to disclose the operation of the mobile station and the base station against the error status.
In the related art, examples of the initial network entry procedure of the mobile station include a spontaneous initial network entry procedure in which the mobile station spontaneously accesses the network or the base station having good channel environment, and an initial network entry procedure in which the mobile station performs the initial network entry procedure due to restart of the base station in a normal operation state registered in the base station.
In case of the spontaneous initial network entry procedure of the mobile station, the number of mobile stations which perform spontaneous initial network entry and a timing point when the network entry procedure is performed are randomly provided. In case of the initial network entry procedure of the mobile station due to restart of the base station, the number of mobile stations which perform the initial network entry and a timing point when the network entry procedure is performed can be predicted and can be concentrated in a moment.
Furthermore, the mobile station performs random backoff to avoid collision with another mobile station during initial ranging for the initial network entry procedure, and the base station forwards a backoff window value for the random backoff to the mobile station through the uplink channel descriptor (UCD) message.
Meanwhile, a broadband wireless access system based on IEEE802.16e system supports a power saving mode for minimizing power consumption of the mobile station. The operation of the mobile station in the power saving mode is performed by repetition of a sleep interval and a listening interval. The length of the sleep interval and the listening interval, which is determined by each value of a sleep window and a listening window, has different values depending on characteristics of traffic set in a corresponding mobile station. Accordingly, the mobile station can have three types of power saving classes as follows depending on characteristics of traffic which is currently set.                Power Saving Mode Class of type 1        Power Saving Mode Class of type 2        Power Saving Mode Class of type 3        
Class 1 targets a best effort (BE) having characteristics of an existing Internet traffic or a non-real-time variable rate (nrt-VR), and is defined by an initial sleep window, a final window base, a final window exponent, a listening window, and a start frame number for sleep window.
Class 2 targets VoIP or a real-time variable rate (rt-VR), and is defined by an initial sleep window, a listening window, and a start frame number for sleep window.
Class 3 is for a management message, such as DCD/UCD and MOB_NBR-ADV, to be periodically forwarded to the mobile station which is in a power saving mode, or data to be forwarded in multicast, and is defined by a final window base, a final window exponent, and a start frame number for sleep window.
Meanwhile, in a state that any one of the above classes is activated and converted into a power saving mode, the procedure of performing handover of the mobile station will be described as follows. For reference, FIG. 6 illustrates a signal flow between the mobile station and the base station to illustrate the procedure of performing related art handover in a power saving mode.
The mobile station of the power saving mode does not receive a downlink signal forwarded from a service base station to minimize power consumption for a sleep interval which corresponds to an unavailable communication interval. Also, the mobile station does not transmit uplink traffic to the service base station. On the other hand, the mobile station can receive downlink data from the base station and transmit uplink data to the service base station for a listening interval which corresponds to an available communication interval.
The mobile station can perform scanning procedure for retrieving channel status of a peripheral base station for a sleep interval which is an unavailable communication interval (S601). At this time, if the peripheral base station uses a frequency band equal to that of the service base station, the mobile station can perform the scanning procedure for the corresponding base station for a listening interval as well as a sleep interval.
As a result of scanning of the peripheral base station, if a condition for handover is satisfied, the mobile station transmits a sleep request message (MOB_SLP-REQ) to the service base station to deactivate all the power saving classes (S603). The service base station which has received the sleep request message from the mobile station transmits a sleep response message (MOB_SLP-RSP), which indicates deactivation of all the power saving classes, to the corresponding mobile station in response to the sleep request message (S605).
The mobile station which has received the sleep response message (MOB_SLP-RSP) deactivates all the power saving classes which are currently activated (S607), and transmits a handover request message (MOB_MSHO-REQ) which requests handover to another base station, to the service base station (S609). The service base station which has received the handover request message (MOB_MSHO-REQ) transmits a handover response message (MOB_MSHO-RSP) to the corresponding mobile station in response to the handover request message (MOB_MSHO-REQ) (S611).
The mobile station which has received the handover response message (MOB_MSHO-RSP) forwards a handover indication message (MOB_HO_IND) to the service base station to request release of connection with the service base station (S613). Afterwards, the mobile station performs network re-registration procedure with handover target base station to perform handover (S615).
In the above steps, the mobile station of the power saving mode may omit the steps S603 and S605 and end the power saving mode. At the same time, the mobile station may transmit the handover request message (MOB_MSHO-REQ) to the service base station to perform handover to another base station.
In the related art, when the mobile station of the power saving mode performs handover, the mobile station deactivates all the power saving classes which are activated and performs handover procedure. At this time, a problem occurs in that the related art fails to suggest how to maintain and update information of the deactivated power saving classes after handover.
Meanwhile, if the mobile station moves from a specific mobile communication region to another mobile communication region, handover which automatically converts a communication channel to prevent communication from being disconnected is performed. The general procedure of performing such handover will be described as follows. FIG. 7 is a flow chart illustrating the procedure between a mobile station and base stations to sequentially illustrate the general procedure of performing handover according to the related art.
The service base station transmits handover triggering information to the mobile station through a downlink channel descriptor (DCD) for transmission of channel information or message (MOB_NBR_ADV) for transmission of information of neighboring base stations (S701), wherein the handover triggering information designates trigger conditions of various operations of the mobile station with respect to handover, i.e., handover related operations. One example of lists included in the handover triggering information is as follows.
TABLE 4LengthNameType(1 byte)ValueType/Function/54.11See Table 358b for descriptionActionTrigger value54.21Value to be compared withmeasurement metric value to decidetrigger conditionsTrigger54.31Averaging duration of metricaveragingmeasurement value in mobiledurationstation
Also, the handover triggering information may further include the following lists.
TABLE 5NameLengthValueType2 bits0x00: CINR metric(MSB)0x01: RSSI metric0x02: RTD metric0x03: ReservedFunction3 bits0x00: Reserved0x01: metric of neighboring base station is greaterthan absolute value0x02: metric of neighboring base station is smallerthan absolute value0x03: metric of neighboring base station is relativelygreater than metric of service base station0x04: metric of neighboring base station is relativelysmaller than metric of service base station0x05: metric of service base station is greater thanabsolute value0x06: metric of service base station is smaller thanabsolute value0x07: ReservedEnabled3 bits0x00: ReservedAction(LSB)0x01: MOB_SCN-REP message response aftereach scanning interval to trigger0x02: MOB_MSHO-REQ message response totrigger0x03: mobile station initiates scanning ofneighboring base station by transmittingMOB_SCN-REQ message in response totrigger0x04~0x07: Reserved
Afterwards, if the mobile station triggers scanning initiation of a neighboring base station to a service base station as a predetermined metric received from the neighboring base station satisfies scanning initiation condition (0x03 of Action list) of the handover triggering information (S702), the mobile station transmits a scanning request message (MOB_SCN-REQ) to the service base station, and the service base station allocates a predetermined scan duration and at the same time transmits a scanning response message (MOB_SCN-RSP) to the corresponding mobile station in response to the scanning request message (MOB_SCN-REQ). The mobile station synchronizes a downlink signal with the neighboring base station for the allocated scan duration and measures quality level of the signal received from the neighboring base station, strength of the signal, transmission synchronization, etc. to perform scanning of the neighboring base station (S703).
If the mobile station triggers scan reporting to the service base station as the scanning result satisfies scan report condition (0x01 of Action list) of the handover triggering information (S704), the mobile station transmits carrier to interference and noise ratio (CINR), received signal strength indication (RSSI) and round trip delay (RTD), which are measured through the scanning, to the service base station through the MOB_SCN-REP message.
If the mobile station triggers handover request to the service base station as the scanning result satisfies handover condition (0x02 of Action list) of the handover triggering information (S705), the mobile station transmits the handover request message (MOB_MSHO-REQ) to the service base station, and the service base station transmits the handover response message (MOB_MSHO-RSP) to the corresponding mobile station in response to the handover request message. Subsequently, the mobile station transmits the handover indication message (MOB_HO-IND) to the service base station to request release of connection with the service base station (S706) and then performs the network re-registration procedure with the handover target base station.
Meanwhile, a broadband wireless access system based on IEEE802.16e system supports a power saving mode for minimizing power consumption of the mobile station. The operation of the mobile station in the power saving mode is performed by repetition of a sleep interval and a listening interval. The sleep interval and the listening interval are determined by each value of a sleep window and a listening window. In this case, the sleep interval and the listening interval may have different values depending on characteristics of traffic set in a corresponding mobile station. The power saving mode can be classified into power saving mode class of type 1, power saving mode class of type 2, and power saving mode class of type 3.
The mobile station which is being operated in the power saving mode can perform scanning of the neighboring base station even though the mobile station does not receive and transmit the scanning request message (MOB_SCN-REQ) and the scanning response message (MOB_SCN-RSP) from and to the service base station. If the mobile station transmits the scanning report message to the service base station whenever performing scanning of the neighboring base station, a problem occurs in that power consumption of the mobile station is inevitably required unlike intention of the power saving mode. However, if the base station does not receive the scanning report message for handover from the mobile station, a problem occurs in that the base station cannot control handover to the mobile station of the power saving mode appropriately.
Furthermore, a problem occurs in that the current mobile station fails to effectively perform handover related operation and power saving operation as the mobile station does not use characteristics of each class by performing handover related operation regardless of the power saving class which is being operated.